The traxxas carrier/drive dog can be found on this page, with some other transmission parts.
www.rc-monster.com/products.php?cat=48&pg=2 (if your going to get the coupler that the other guys are talking about then you wont need the traxxas one)
If your going to be pulling apart and replacing parts in the transmission then I'd probably replace 2nd gear as well for the sake of $3 (underneath the drive dog on the page I linked to).
I'd get a steel idler as well (you only need one if your going to use 1 speed)
http://www.rc-monster.com/proddetail...03idler&cat=48